DEC. 1534
RcHig-PROCEEDINGS
lab. v.
Scheepsbouwkundf
Tethnisd.,e Hotitschoo!
CcIt
P1975-7
Volume 4
THE SYMPOSIUM WILL BE HELD IN THE NETHERLANDS,
THE HAGUE - CONGRESS CENTRE - 27-31 OCTOBER 1975
Statements and opinions expressed in the papers are those of the authors, and do not necessarily represent the views of the Royal Netherlands Navy.
The papers have been reproduced exactly as they were received from the authors.
VOLUME 4
SESSION LI.Chairman: P. Slijp
Captain R. Neth. N.
Head of the Mechanical Engineering Department., Ministry of Defense (Navy)
Propulsion control systems to meet the requirements of small vessels.
R.A. Toyne.
Control of superconducting marine propulsion machinery. A.D. Appleton and T.C. Bartram.
The control of power injection for the cruiser shore trials facility at Ansty.
J.P. Cleland, A.W.J. Griffin and N.P. Lines, SESSION L2:
Chairman: J.B. Spencer
Ship Department, Ministry of Defense, (Procurement Executive), Bath. Application for optimal control theory to a large hydrofoil craft.
S.K. Hsu.
Determination of stabilizing fins for swath ships. C.M. Lee and M. Martin.
Control simulation of air cushion vehicles. Z.G. Wachnik, R.F. Messalle and J.A. Fein.
The incorporation of fan dynamics into the motion simulation
of surface effect ships.
J. Schneider and P. Kaplan. SESSION MI:
Chairman: N.H. Norrbin
Statens Skeppsprovningsanstalt, GOteborg Ship type modelling for a training simulator.
C.C. Glansdorp.
Identifying the marine vehicle from the pulse response.
T.B. Booth
Applications of digital simulation analysis to ship control .
dynamic positioning control of drilling ships. H. Eda.
Mathematical modelling of ships.
J. van Amerongen, J.C. Haarman, W. Verhage.
Page 4-1 4-15 4-28 4-46
4,58
4-73
4,91
4-117
4-137
4-151 4-163SESSION M2,
Chairman, A.G. Boiten
Professor in control engineering, Department of Mechanical Engineering, Delft University of Technology
A programmable electronic controller for gas turbine propulsion systems.
R. Kendell.
The use and experience of hydraulic fuel systems in the control of marine propulsion gas turbines.
H. Saville and D.J. Wheeler. Integrated turbine control.
B.D. Taber.
Electronic based power control systems for gas turbine propelled ships.
M.J. Joby and S.G. Perring.
Page
4-179
4-192
4-209
4-226/240
PROPULSION CONTROL SYSTEMS TO ME M THE REQUIREMENTS OF SMALL VISSELS
BY
A.A. TOYNE
Regulateurs Europa Ltd.
SYNOPSIS
There is an increasing number of small ships being built for
specialised
applications. Some for use in the military sphere and others for commercial
use; notably fast patrol boats and oil rig tenders etc. The basic service
requirements of
each
individual vessel together with the characteristics ofthe propulsion equipment employed, results in a propulsion control system which is unique to each vessel type.
This paper describes a
simple
basically standard electronic propulsioncontrol system, which has been developed to be readily tailored to meet the requirements of most bridge/multi-station control applications where diesel propulsion is employed.
In design time and initial cost it offers considerable savings and in addition provides a high degree of flexibility.
Where controllable pitch propellers are specified, the furnishing of a combined pitch/speed programme in the early stages of the contract may be relegated in importance, and if necessary, the programme may be set up in the light of trials information.
A first generation of these systems is in use, and the paper describes a
typical system. The importance of functions that can be provided by the engine
speed governor are highlighted, and its future role and that of the systems discussed.
INTRODUCTION
Remote control of propulsion machinery is an integral part of almost all
ships both large and small, built in recent years. It is almost essential for
a degree of automation to exist in any system to relieve the remote operator of some of the responsibility for safe and correct operation of the propulsion machinery.
The advantages of remote control are well known. Major ones such as the
reduction in the number of trained engineers required and the improved response rate brought about by dispensing with the intermediate link of telegraph and
engine room staff are of great importance on small ships. The latter advantage
is of particular relevance in some of the more specialised vessels where precise control of speed and direction are essential.
In small ships the reduction in the number of trained engineers, occasioned by the use of a propulsion control system may, in itself, create
problems. Those engineers who are carried will probably be mechanically
orientated, and thus not qualified to maintain a sophisticated control system. Pneumatic systems have tended to score in this area due to their basic
simplicity and ease of appreciation by engine room personnel.
However, many factors favour electronics for propulsion control systems. Weight, size and speed of response are only some, and one more overriding
influence is the absence of a ready supply of compressed air. This is fairly
common now that electrically started high speed diesel engines are more extensively employed.
With the above points in mind, the following guide linos were laid down for the development of a range of electronic plug-in circuits which could be used to form a propulsion control system suitable for most small diesel prepelled
vessels:-(a) Flexibility applicable to a wide range of vessel types
(1)) Low Prime Cost - the minimum of time spent in design and manufacture
to meet the requirements of a specific application.
Reliability simplicity of design without undue sophistication.
Maintainability - repair by replacement of electronics with clearly
defined functions. Cost of circuit cards to be
sufficiently low as to encourage carrying of spares
(0)
Minimum of Use of electrical signals for all but load controlinterfaces function.
In terms of cost effectiveness, parity with simple pneumatic systems was used as a top limit, with an estimated saving of approx 75% in installation costs.
Th. engine speed governor was seen as an area Which could be further developed to be directly compatible with electronic systems and which could carry out a number of functions mechanically, which would otherwise have required electronic solutions.
....B.rldge Power Lever Speed Limit E / R Speed Lever E /R Pitch Lever False Idle
T
Speed Pitch Speed PitchFig 1
Hypothetical System
Governor Output Comparator
Enue 1
4-4 PROPULSION CONTROL SYSTEM
Principle decisions taken in the light of past experience were
that:-(a) Voltage analogue techniques would be used. (b) Systems would cater
for:-Controllable pitch propulsion
Fixed pitch propulsion with shaft brakes Fixed pitch propulsion without shaft brakes
(c) Single lever control would be employed in order to give an immediate
feel of power setting without consulting instruments.
(d) Interlocks would be kept to the minimum and the human link employed
where it was considered advantageous.
(e) Redundant features would be largely eliminated by employing electronics
as the nucleus of any system with the switching logic, specific to an
application, performed by relays.
(f) Plastic film potentiometers would be used for inputs and position sensing in preference to linear variable differential transformers on
cost effective grounds. Experience had proved both to be perfectly
satisfactory.
(g) Fail safe principles would be employed wherever possible or easily
defined, with particular attention paid to power supplies and voltage
level drift.
(a)
Plug in printed circuit cards would be used. Past experience withmodules had shown them to be approx four times as expensive to produce
and more difficult to maintain or fault find. In addition printed
circuit cards would be compatible with existing speed switch, load
share circuits etc. Typical System
The basic requirements for the electronics, as seen, are Shown in the
hypothetical system depicted in fig. 1. This dhows a twin engined vessel
driving, via a gearbox, a single controllable pitch propeller. Control is via
two command levers in the engine room or machinery control room. One for pitch
setting and the other for speed. A linear relationship between lever movement
and output signal with a dwell in neutral is given by both of these levers.
Control from the bridge is by means of a single power lever. This mode of
control is to a pre-set pitch/speed programme of the type shown. A pitch
limit may be invoked when operation with only one clutch engaged and a speed
limit is shown which may be required, for instance, to prevent propeller
cavitation on fine pitches.
In order to assist with 'bunpless' command transfer between control
stations, matching comparators are incorporated which drive either matching
meters or lamps showing the state of match. As it is normal practice for the
bridge/engine room selector to be housed in the machinery control space it is
the responsibility of the engineer to align his levers with the setting of the
bridge combined pitch/speed lever in order to affect a smooth transfer.
Pitch
10 Astern--
_\
,
./
... .s.,
.,'
.
...s N.,,,,,..
.../
,
../
r" N
.
,
N..s. N s., N. N .../
,
\
NN N.,
IX N
N, /
N NI'
S., .",\
N, NI
IsN
N.N N
NN ,
1 i N N N N N'
IN N N
i N,
Y I, N N X
N
IL. X
\._ >'
Speed V Typical ProgrammeFig 2
Pitch & Speed Programmes
.--
.---c
-c-./ \
\,
N,
NN,
N.'NI
..'
.N.N
I %,N N
,N N
N.N N N
1/,
`,...\,
.N. N).1-.N. N.
N,./
N
-/NN 5NN \
, N N '
N,
i,N
Pose. Ahead 10 Typical Programmeincorporated. It is a subject for debate, but practical experience has favoured utilizing the human link, with its ability for applying discretion for performing this function.
Each command lever is merely a drive unit for a potentiometer, the potentiometer giving a linear output irrespective of the programme required, A voltage analogue signal derived from the command levers is shaped in each case by a programming circuit, with a single voltage signal being used for pitch and
speed programmes from the bridge command unit.
The selected programmed signal for speed is used as the input for the speed
setting switching servo loop of both engines. The dead band being the minimum
required to prevent instability.
A governor output comparator is fed with voltage analogue signals derived
from potentiometers on the governor output Shafts. Providing that the
governor/engine fuel pump relationship is correctly set this signal is
representative of the load on each engine. The signals thus displayed on
differential meters may be used as a reference, and the load shared between engines using a speed trim which modifies the speed setting feedback for one
engine only. Automatic forced load sharing was not considered necessary between
two engines operating with droop governors, but could easily be added for greater numbers or where Shaft generators required small droop settings.
The governor output comparator is also used to automatically detect the
highest
loaded engine and by means of a solenoid valve, ensure that the load control loop is composed of the pitch actuator and load control valve of thegovernor on the highest loaded engine. To prevent slight load instability from
causing the load control function to be alternated between engines, the circuit is such that control of load is only taken or relinquished by an engine when its share of load is outside a pre-set band.
The pitch setting loop is similar to that for speed, utilizing the pitch
setting solenoids and true pitch feedback potentiometer. With neither of the
clutches engaged the pitch signal is set to drive the propeller pitch to the zero thrust position.
As depicted in fig. 1 the system is aimed at controllable pitch propeller
applications. To utilize the same circuits for fixed pitch vessels the
following two further facilities are incorporated in the speed setting circuit.
(a) A false raising of the idle speed to prevent engine stall on clutch
engagement and (b) clutch engage signals triggered at pre-set levels of the
speed analogue signal. (This was done in preference to using micro switches
on the command levers in the interest of maintaining a simple standard command
unit.) To ensure that the neutral position is always maintained, the potential
at the neutral point of the command levers is tied by a centre tap to a
stabilised power supply at the same potential. This supply being used as a
datum in all other parts of the circuit. Pitch and speed programme
In order to achieve the correct pitch/speed relationship on C.P.P.
applications it is necessary to programme the circuit cards containing the pitch
and speed loops. It is possible to do this at any time and the programmes may
be adjusted on sea trials. This has already proved an advantage as hitherto
reliance upon the shipbuilder to obtain and correlate engine, propeller and hull data, has often resulted in the pitch/speed relationship being the holding item
to design completion. This is particularly relevant to pneumatic systems
where design and manufacture of special cams are required.
Speed Setting Shaft
Feedback
Piston
.41T7
Lower Speed Solenoid
Fig 3
Speed Setting Mechanism
Raise Speed Solenoid
Fig 2 shows a typical propeller pitch and engine speed programme, plotted
as voltage on the vertical axis against command lever movement. The shaded
area in each case represents the envelope of adjustment available to cater for
varying applications. The pitch programme is capable of giving two different
slopes in the ahead and in the astern direction with the capability of a dwell
zone at any of the five transition points. This is achieved by feeding the
command lever signal via an emitter follower, to four operational amplifiers. The switch on point of each amplifier, set by adjusting its bias, gives the
start of each slope. Gain and saturation point adjustments give the facility
to set the angle of each slope and its maximum value, respectively. By cross
linking, negative slopes are possible.
The speed programme as depicted has similar adjustment facilities, but in
this instance only one slope is possible for each direction with dwell zones adjustable from neutral and at the maximum set speed levels
THE ENGINE SPEED GOVERNOR
To utilize these systems in conjunction with medium and high speed diesel
engines it was necessary to have available a compatable governor with a speed
setting mechanism that could form part of the speed setting loop, and which would
give a speed setting response which was similar to that obtained using a
pneumatic speed setting servo.
The most obvious choice was a conventional hydraulic governor that would
be capable of governing the majority of diesel engines encountered in small
ships. Available were various electrical means of speed setting, Speeder
motors, stepped controllers etc, but none giving the required response rate or
infinately variable capability demanded by propulsion engines.
An external electro-hydraulic speed setting mechanism was considered, but
dismissed on the basis of being cumbersome and expensive with the requirement
of engineering to each application. Instead an integral speed setting
mechanism was designed, using the high pressure oil developed by the governor
gear pump. This latter solution provided a package Which was unlikely to
cause installation difficulties.
A diagramatic arrangement of the speed setting mechanism is Shown in Fig
3.
As in the standard governor, set speed is adjusted by rotating the speedShaft.
An increase speed input to the speed setting loop causes solenoid A to
be energised, moving its associated pilot valve down. Side C of the speed
setting cylinder is put to drain and pressure on side D causes the piston to
move to the right. Via a bell crank lever, this moves the speed setting shaft
anti clockwise to increase speed. A linear potentiometer is used to provide
feedback to the speed setting comparator and solenoid A is de-energised in the
matched condition. The rate of speed increase is controlled by a restrictor
valve and a flow regulator. The latter acts to maintain a maximum pressure
drop across the restrictor valve. This is necessary to deal with oil viscosity
changes between hot and cold engine conditions.
Pitch Limitation
Shown in fig 4 is the load control valve
(Lev)
which is part of thegovernor when used for controllable pitch applications. The position of the
valve is determined by a combination of governor speed settingand governor
output. Adjustment of the range and datum settings will cause the valve to be
ma
pivot
Power range adjustment
speed setting shaft
Eig.4.
Load Control Mechanism
max Engine Speed
Fig5. L.C.V. Characteristics
11,decrease
governor output pitch
increase
Raise
Lower
a
r.7
Fig.,6. Speed
Limitation
4-10
from gov. supply Engine R.PfC Fig. 7.Characteristics
flow control valve 5 max dielapped at a unique load level for any set speed. The characteristic being a straight line which approximates to the engine power/speed curve (fig. 5). When incorporated into the propeller de-pitching mechanism the propeller pitch
is prevented from passing beyond this line and thus overloading the engines.
Speed Limitation
The governor load control valve may be used for fixed pitch application also, to limit the set speed increase rate such that a governor output position
limit curve is not exceeded during engine acceleration. When the governor
output is below the limit shown in fig 7 the WV will be below the lapped
position depicted in fig 6. Speed control is then as described earlier. A
rapid increase in speed demand will be permitted until the governor output limit
line is reached. At this point lapping of the WV ports prevents further speed
increase until the governor is below the limit line. Similarly if the load
increases beyond the set limit when running at a steady speed the WV will move
up through the lapped position and bring about speed reduction. This is
particularly relevant to sprint rated vessels and those subject to large steady speed load changes.
FUTURE REQUIREMENTS Governor
With progress in the high speed diesel engine field towards more highly rated turbocharged engines, there is a need for fuel limitation related to boost pressure to reduce engine wear, black smoke etc.
This
will probably be incorporated simply, into future governors used withfixed pitch and controllable pitch propulsion systems. Fig 8 shows the
principle which is already developed and in use on industrial and traction
governors. The normal governor action is inhibited if the boost pressure is
insufficient for the fuel demanded. The governor pilot valve is acted upon by
the flyweights and a variable stop,
which
is positioned by a boost pressure servoand the governor output, such that the boost pressure operates as an overriding inhibition to increasing fuel.
A typical relationship of boost pressure to governor, output is shown in
fig 9. The step decrease in the vacuum condition is to cater for complete
turbocharger failure.
In controllable pitch propeller applications where load control is featured the load control valve may be mechanically interlinked with the fuel limit such
that its operation is related to boost pressure. The characteristics would be
as shown in fig 9, with the absence of required turbocharger pressure causing
de-pitching rather than stalling down of engine speed. Fuel limitation in this
case would act as a secondary protection.
Systems of the type described have, to date, used hydraulic control of load. This offers the advantage of being readily appreciated by mechanical engineers. However, to eliminate this interface, it is quite possible to fit a linear variable differential transformer in place of the load control valve, and use
its output to attenuate the set pitch signal. This arrangement has been
successfully employed in conjunction with an electronic propulsion and steering
control system on a double ended ferry with Voith Schneider propellers. In
this case proportional two dimensional pitch unloading was used to maintain ships heading
Boost Pressure Increase
Speed Increase
Fig. 8. Boost
Limit Mechanism.
Boost Pressure p s
Fig.9.
Characteristics .
4-12 Power Piston0
10 9- 8- 6-5 4 3 2 0 Otnt foe' 001(01 10 20Systems
The functions that electronic systems are capable of performing are almost
limitless. The systems briefly discussed in this paper are just one approach
to providing cost effective propulsion controls for small ships with the aim of
maintaining the basic requirements of simplicity and reliability. They have
been evolved in the light of experience of similar propulsion controls using various media, and future development will continue upon this line.
The use of voltage analogue methods may be subject to debate, but in practice this technique has proved satisfactory, providing attention has been paid to using high impedance loads and a degree of signal line screening to prevent pick-up.
As with larger vessels it is possible that future systems may employ a computer as the heart of the propulsion control and surveilence equipment. However, it appears that at present this would not be a practical possibility for the majority of the class of vessels under discussion.
As a company we are largely dependant upon feedback of information from users to determine whether the correct approach has been employed and also where
further attention should be focused. In this respect it is fortunate that a
considerable number of similar systems will be in service. It is the intention
to closely follow the service record of these vessels.
Further development will also proceed in the following areas.
Command Units
Consider use of redundant circuits for critical areas.
Attempt to define more clearly 'fail-safe' and consider where 'fail soft' would be a more viable proposition; bearing in mind the
influence of external factors upon what is 'safe'. Consider the use of more self monitoring and provision for self fault identification. Rationalise design of command levers with the aim of standardizing on one design that would be acceptable for use in exposed and sheltered
positions. (There are so many different forms
of command inputs used on ships. It would seem
an advantage if some universally acceptable guide lines were laid down for tho form that they should take, as in the case of controls in motor cars etc)
Consider the use of wandering lead control for use on bridge wings.
Evaluate in economic and practical torme as to when it is more viable to use mechanical interconnection of bridge stations as against electrical command transfer.
(c) Emergency Control Design standard arrangement for emergency
over-ride in case of failure.
Consider grouping of emergency manual controls in the engine room or H.C.R.
FXPERIENCE IN SERVICE General Observations
At the time of writing approximately 30 of the systems discussed have been
manufactured, with over half of that number in service. Only two of the
vessels have been equiped with controllable pitch propellers. The remainder
being twin engined fast patrol vessels with fixed pitch propellers and no shaft
brakes. On the latter vessels, to ensure satisfactory manoeuvring and crash
stop operation a 'fuel on' detecting switch is built into the governor. On
reversals engine speed setting is reduced to idle and the clutch is maintained in engagement in the original sense, until the trailing propeller allows the
actual engine speed to drop to the net level. Reversal is initiated by the
'fuel on' switch sensing that the engine is once again driving the propeller. Typically 10 second reversals from speeds in excess of 25 knots have been achieved.
Problems have been mainly of a minor nature with inevitable pre-trials installation errors predominent, and a small amount of infant mortaility of
electronic components. Wiring errors have been mistakenly attributed to
faulty operation of the system, and in this area the addition of more self monitoring points would alleviate the need for expert assistance at the pre-trials stage.
No faults have been reported in service and it is hoped that should they
occur, the area of fault could be readily identified, as each functioni.e.
pitch loop, speed loop, function limit, stabalised power supply etc. is contained on a *operate circuit card.
The plastic film potentiometers used to derive voltage analogue signals from command lever units were found to suffer from tracking problems, due to
condensation, when used in exposed control stations. This was cured by
encapsulating the terminals.
Modifications
On all vessels emergency control is available locally at the machinery. Engine speed can be raised or lowered by manual push buttons operating onto
the top of the governor solenoids. However, an addition to some later
vessels was the inclusion of an emergency mode of bridge control. This was
composed of centre biased switches for engine speed control and three position
switches for clutch operation. A key operated emergency mode switch was used
to by-pass the system logic.
Various styles of command lever tops were tried but the traditional
telegraph styles were favoured. Alterations were made in this respect on some
vessels and also the neutral band was extended and given additional feel to allow for operation in adverse environments.
4TH SHIP CONTROL SYSTEMS SYMPOSIUM ROYAL NETHERLANDS NAVAL COLLEGE
27 - 31 OCTOBER 1975
CONTROL OF SUPERCONDUCTING MARINE PROPULSION MACHINERY by
A.D. Appleton and T.C. Bartram
THE SCOPE FOR SUPERCONDUCTING PROPULSION MACHINERY
Superconducting d.c. motors and generators have been under development at International Research & Development Co. Ltd., for about 11 years and
a number of machines have been produced. Details of the machines and
their general design features have been published1'213 and it is not
necessary to repeat the description here. However, it is useful to
examine the range of machines which this new technology brings before we proceed to discuss the control aspects of their operation in marine applications.
There are always limits to which a certain design approach may be extended and, commonly, the first limit to be reached is an economic one; it is generally possible to make further improvements by spending sufficient money but this may have negative cost benefits and therefore he difficult
to justify. However even in situations where financial restrictions do
not exist, there are always limitations and one of the first casualties
may be reliability. The availability of the superconductor makes it
possible cc create radical changes in electrical machine design and this comes about because superconductors (of the right type) can carry very
high current densities without energy dissipation. One of the consequences
of this is that it becomes possible to eliminate the iron magnetic circuit
and immediately more space is available for copper conductors; it is also
consider the benefits
which
superconductivity bring to marine propulsion systems, let us consider briefly some of the more important design aspects of the machines in general.A starting point for the deign of a conventional type of d.c.
machine, is the simple equat1on4.
k D2 L
Where P is the machine power; kW
is the speed; rev/min
is the diameter of the rotating armature; metres
is the length of the armature; metres
k is a constant
The value of k will vary slightly according to the materials selected
and also depends upon the size of the machine. For large motors in the
megawatt range, k is about 6.0.
If we select a rating of 40 MW at 60 rev/min, such as might be required
for a tanker drive motor, the necessary value of D2L is 83 cubic metres and
the total motor weight is of the order of 1000 tonnes and is quite impractical.
Previous studies5 have indicated that the maximum rating of conventional
heteropolar d.c. motors is about 10 MW and even then over a rather limited
speed range of about 70 to 200 rev/min; outside this speed range the power
rating is reduced. The possible ratings for
superconducting d.c. machines
are in excess of 200 MW over a wider speed range.
The voltage which may be developedbetween one pair of sliprings in
a drum type motor, indicated schematically in Fig.
1, could be 80 volts
at a speed of 80 rev/min (a value quite impractical for an
iron cored
homopolar machine). For a terminal voltage of about
1.2 kV, it is clear
that we will require 15 pairs of sliprings and a current
of about 33,400 amperes;
this is not an unreasonable first approximation to the design but obviously
there is great scope for design variation.
Perhaps the most difficult problem of all with homopolar machines is current collection, and it has been studied by a number of people for many
years. A popular solution is to use liquid metals but IRD decided to seek
an alternative; the requirements call for a performance beyond that
possible with conventional brushes and one of the most important developments to emerge from rRD,s work in this field is the metal plated carbon fibre
brush, some details of which have been published6'7'8. The performance of
this brush is very good with current densities of 100 A/cm2 being achieved with good voltage and wear characteristics.
Returning to our selected design and choosing a conservative current density of 80 A/cm2 we see that a total brush area of 416 cm2 is required. A convenient width of slipring might be about 3 cm so that about 140 cm
of the circumference of the slipring needs to be covered with brushes; this is no problem for a slipring whose diameter is about 3 m.
This relatively simple design discussion is sufficient to show that a motor of the selected rating is well within the possible limits of the new
machine. It can also be shown in a similar manner that the design of a
generator running at a higher speed is a relatively straightforward procedure.
The provision of the helium refrigerators for the superconducting machines presents few problems provided that high engineering standards
are adopted with particular care to the choice of the compressor. It is
desirable to avoid the use of long liquid helium transfer lines and to ensure that adequate redundancy of critical items is included.
It is not possible in a single paper to cover the design of the machines and the more critical problem areas in detail and reference should be made to the literature.
Turning specifically to the application of superconducting d.c. machines to commercial ships there is at least good qualitative
evidence to suggest that economic benefits will emerge. An essential
point to note is that superconducting d.c. motors and generators have now reached a point where their commercial exploitation is possible.
What are the benefits that superconducting propulsion systems can
offer? In two words - flexibility and reliability. Fig. 2 shows a
comparison between a diesel engine of 29 000 hp and 100 rev/min, and weighing 1380 t, compared with two medium speed diesel engines of 15 000 hp and 450 rev/min each driving a superconducting direct drive
propulsion motor. The weight of the latter system is
Diesel engines 160 t each 320 t
Superconducting generators 55 t each 130 t
Superconducting motor 150 t
An alternative scheme would be to employ three diesel engines of 10 000 hp each to give even greater flexibility.
Fig. 3 indicates how any number of smaller power units may be
employed to drive one, two or any number of shafts. The benefits may
be summarised as follows:
The space required for machinery may be reduced to a minimum; this is already important for liquefied gas carriers and roll
on-roll off and container ships. It is also important for tankers
now that large clean water ballast capacity are going to become
mandatory.
Any form of prime mover may be employed, and a compact generator
unit consisting of a prime mover/superconducting generator set
may be located in any part of the ship.
The total ship power requirements may be provided from standard
prime mover/generator modules factory-assembled and tested
complete on bed-plates before being installed in the ship. In
addition, the prime movers, generators and control system can
be tested before the ship goes to sea; the motors can be tested
at full load current (at low voltage).
The installed electrical capacity may be employed for the discharge
of cargo in port; the power demand for the latter could be as high
as 10 MW and it would be unnecessary to provide additional auxiliary
The reliability of the propulsion system may be increased because any one of a number of power units may supply the
propulsion motor(s). Any number of engines may be used and
with only one running it is still possible to maintain independent speed control for a twin screw ship.
Reversing gearboxes and indeed all gearboxes may be eliminated. The control characteristics are such that reversal may be
effected very quickly without the use of the expensive controllable pitch propeller.
The superconducting motor may be readily designed to suit contra-rotating propellers which normally require complex mechanical drive systems.
The IRD design of machines allows the use of medium voltages
Cl or 2 kV) so that it is unnecessary to transmit very high
currents.
Brief studies of non-conventional hull arrangements indicate that there is an advantage in being able to locate components of the propulsion system in different parts of the vessel.
It is possible to obtain constant-frequency a.c. power for auxiliary load of the ship directly via the main propulsion generator and thus eliminate at least some of the auxiliary diesel generator sets.
The flexibility allows engines to be out of service and repaired at sea while still retaining some propulsive power.
The motor speed can be trimmed to suit optimum propeller speed and hence reduce fuel consumption.
No. of Available
; Generators
Power 100%
Table 1.
This feature may be particularly beneficial to
warships which spend
much of their operational lifetime considerably below
the installed
power capability.
4-20
Max. Speed of Motor
Propeller % Excitation %
DISCUSSION ON CONTROL ASPECTS
An important feature of d.c. electrical transmission is the ability
to employ a number of prime movers with one propeller shaft with the resulting flexibility of being able to operate as many (or as few) of the
installed engines as are available or necessary. This effectively utilises
the available power by giving the equivalent of a variable ratio gear box.
For example, consider an installation comprising 3 generators supplying one
propulsion motor, the generators being connected electrically in series.
At full power each generator has output V and I and the motor
conditions are 3 V and I at full speed, full power. With 2 generators
2/3 power is available at a total of 2 V applied to the motor.
In order that the propeller may absorb 2/3 power, its speed must be
approximately 87% full speed (cube law assumed) so a reduction in motor
excitation to 76% will give matched conditions.
Similarly
with
one engine on 4./3 power,this power may be absorbed
at 69% full speed and with a motor excitation of 48%.
This gives three effective 'gear' ratios allowing prime movers to
be used at maximum capability when reduced in number,
regardless of their
individual characteristics. A summary is given in Table 1.
3 100 100
100
2 66.7 87
76
In some circumstances it may be beneficial to operate the generators in parallel, possibly to reduce capital cost, in which case the foregoing benefits are not available and the system behaves as for a constant ratio gearbox system.
An attractive system which is available with superconducting d.c. propulsion is where a twin shaft ship is required to he driven by an odd
number of prime movers, for example with 3 prime movers. The design
approach with homopolar machines is to have double armature generators, an arrangement which is extremely simple to achieve, without incurring
any cost penalty. By connecting one and a half machines in series to
each motor the scheme will provide independent speed control down to at
least 1/3 speed, with some limitations below that value; for manoeuvring
in confined waters only one generator per motor would be used giving fully
independent speed control. With electrical transmission on two shafts
it is a simple matter to design the system such that engines normally associated with the port shaft may be used to drive the starboard shaft
and vice versa. Another important feature is that one engine may be used
to drive both shafts for economy during cruise conditions.
The characteristics of marine engines both in use and likely to be available in the foreseeable future are compatible with superconducting
machines except where high speed outputs are involved. In such cases,
for example with gas turbines at 5 to 6000 rev/min a single reduction
gear box would be necessary to drive the generator, without loss of flexibility. Alternatively directly driven conventional a.c, generators, with diode
rectifier converters may he more attractive. In this latter case the
advantages of d.c. transmission are retained with those of the
super-conducting motor. Whichever generating system is chosen the 'variable
ratio' features of the equipment may be used to run the prime mover at
optimum conditions of fuel economy and lifetime. It may also be useful
to optimise the speed of the propeller more closely than is usually possible (because of manufacturing tolerance deviations from its design value) to give maximum possible performance regardless of the hull
condition. This may he done without disturbing the rated speed or other
It is possible to alter the voltage applied to the motor in a number
of ways. The most practical ways
are:-Excitation variation Engine speed variation
Staging i.e. by varying the amount of the total generator armature which is connected in circuit.
Of course combinations of these might well be used in practice.
Some of the benefits are self evident, others need to be quantified
and these studies are in hand; the prospects for superconducting
machinery appear to be good, given assured reliability.
SUMMARY AND CONCLUSIONS
1. Superconducting hamopolar machines are available for industrial
applications because of the intensive development work which has
taken place over the last 11 years. Practical machines have been
constructed and through this experience the
designs
have beengreatly improved.
2. Superconducting d.c. machines are extremely robust, simple in
construction and if one attempts to define the two aspects in which
they are better than any alternatives, the conclusions reached are:
ability to produce extremely high torque drives; the
torque is.high at all speeds (including zero speed)
ability to produce large amounts of d.c. power directly
at a voltage of between 1 kV and 2 kV.
It must be added that in a number of cases, there is no alternative plant available.
3. Of the many applications marine propulsion appears
to be one of the more important and one which is receiving a considerable amount of
attention at the present time. There are numerous benefits which
vary for different types of ships but in all cases there is a
considerable increase in freedom of choice for the naval architect.
Engine Speed Variation
Staging
driven alternators for ships services
Stepless control
Minimum of 'extra' equip-ment required - depending on power levels and speed turndown etc.
Stepless in control band
Full range control available although not stepless
Engines may be run at constant speed - giving drive for auxiliary alternators.
TABLE 2.
Full range control usually not available
Complications arise if shaft driven auxiliary alternators required
Stepped control
Heavy current contactors required.
Heavily dependent on type and number of prime movers.
Unlikely to be used for marine Pur-poses unless as part of a system combined with (i) or (ii) above.
Advantages Disadvantages Remarks
(i) Full range of control easily
provided
(i) Expensive when very rapid
changes are required.
Ideal for large merchant ships e.g. tankers, bulk carriers etc. Also for vessels where good
(i) Excitation
Variation
(ii) Engines may be run at
constant speed - giving an admirable drive for shaft
(ii) Field forcing equipment
ACKNOWLEDGEMENT
The authors wish to acknowledge the support of the Ministry of Defence
and the Department of Industry in the work described; also to thank the
Directors of IRE for permission to publish this paper.
REFERENCES.
APPLETON, A.D. Superconducting Machines, Science Journal, April 1969.
APPLETON, A.D. and MACNAB, R.B., A Superconducting Model Motor.
Commission I. London,
Annex,
1969, I. Bull., I.I.R.APPLETON, A.D. Les Machines Supraconductrices. La Recherche.
Vol. 3, No. 21
CLAYTON, A.E., The Performance and design of Direct Current Machines.
Pitman (1959).
APPLETON, A.D. Motors, Generators and Flux Pumps. Commission I.
London, Annex. 1969. I. Bull. I.I.R.
6. McNAB, I.R. and
Machines. Proc
7. APPLETON, A.D.
Applied Superco
WILKIN, G.A. Carbon Fibre Brushes for Superconducting
. I.E.E. Electronics & Power. January, 1972.
Status of
Superconducting
Machines - Spring 1972.nductivity Conference.
Annapolis,
1972.8. McNAB, I.R. and WILKIN, G.A. Life Tests with Carbon Fibre Brushes.
SUPERCONDUCTING WINDINGS
HIGH FIELD SLIPRINGS P 2s
w
35-q
Dx10-3P Power; kW
N Speed; rev/min
D Diameter of high field slIpring; metres
cl Current collection per metre of sliprIng
circumference; A/m
0 Useful machine flux; kb
2 MEDIUM SPEED DIESEL
ENGINES 320 TONNES TOTAL
29000 HP DIESEL ENGINE 1380 TONNES 100 REV/MIN 4-26 2 SUPERCONDUCTING GENERATORS 130 TONNES TOTAL SUPERCONDUCTING MOTOR 150 TONNES 60 REV/MIN
Fig. 2 COMPARISON OF SLOW SPEED DIESEL ENGINE DIRECT
DRIVE TO PROPELLER AND TWO MEDIUM SPEED DIESEL ENGINES AND SUPERCONDUCTING PROPULSION SYSTEM
DIRECT DRIVE PROPULSION MOTORS
Fig. 3 ARRANGEMENT OF MULTIPLE GENERATORS SUPPLYING
ANY NUMBER OF PROPELLER DRIVE MOTORS INTERCONNECTIONS
MADE AUTOMATICALLY PRIME
THE CONTROL OF POWER INJECTION FOR CRUISER SHORE TRIALS
by
J.P. CLELAND B.Sc PhD A.W.J. GRIFFIN B.Sc PhD. C.Eng.
(Y-ARD Limited,Glasgow)
N.P. LINES B.Sc
(Rolls-Royce (1971) Ltd.)
SUMMARY
This paper describes the project undertaken by Y-ARD Limited and
Rolls-Royce ( 1971) Limited under contract to MOD(PE), to devise and
implement a control system which would be capable of dynamically loading
the CAH propulsion machinery system installed in the CAH Shore Test
Facility at Rolls-Royce (Industrial and Marine Division) Ansty.
The control system adopted has become known as the Power Injection
System, and the paper describes the concept of power injection and its
consequent demands on machinery hardware and control requirements.
The use of computer models of the machinery system has been a major
feature of the project, and the paper describes the use of such models in
assessing the feasibility and controllability of the proposed power injection
system, testing the computer-based control adopted, and assessing
machinerysafety aspects and operational requirements.
The paper also describes the hardware implementation of the power
injection system and discusses the events and problems encountered during
the programmed series of preliminary tests on the major
machinery items
and on the control system.
I. INTRODUCTION Background
The CAH is an Anti-Submarine Cruiser due to enter
service with the Royal Navy in the late
seventies. The propulsion system consists of two fixed-pitch propellers, each being driven by two gas
turbine units through a reversing gearbox, incorporating self-synchronising clutch units, for normal
use, and hydraulic couplings for manoeuvring.
Previous naval experience with reversing gearboxes of this type driving a fixed-pitch propeller
indicated that
the CAH might suffer from manoeuvring problems which were peculiar to the
characteristics of the hydraulic couplings, and that these problems would be compounded by the
adoption of a single-lever bridge-control concept. An extensive simulation study was undertaken by
Y-ARD to clarify the manoeuvring aspects of CAH, with the specific aim of designing a machinery
control system which would (a) allow single-lever bridge control and (b) obtain the best possible
stopping performance within the design constraints of the system. This study showed that the main
manoeuvring problems arose during single engine/shaft crash stop manoeuvres, and that the problems
were all directly related to the characteristics of the hydraulic couplings.
The three main problems are highlighted in Figure 1, which shows the expected behaviour during
a typical single engine crash stop manoeuvre.
Shaft Speed
Power Turbine Speed
Ast. Coupling Temperature
Figure 1
Power Turbine Speed
As the ahead power is taken off and the transmission changes from ahead to astern drive, the
point of re-application of power must be chosen with care to achieve a compromise between high
power dissipation in the filling astern coupling and low power turbine speed. Power re-applied too
soon leads to high temperatures in the coupling; power applied too late leads to low power turbine
speeds and possible reversal.
Coupling Oil Temperature
As the main shaft continues to decelerate at a rate dictated by the applied astern power and the
opposing propeller power, the power dissipation in the astern coupling builds up to a maximum just
prior to reversal, producing an oil temperature transient as shown. Balance between reversing
performance and the limiting coupling oil temperature has to be obtained.
Shaft Stall
Detailed studies of the CAH gearing and shafting friction losses, in conjunction with expected
rates of deceleration down to zero shaft speed, have led to the conclusion that, for the astern power
levels available, the main shaft may stall during manoeuvres from high ahead speeds. Such an event has
the following results:
All engine power is dissipated in the coupling, thus increasing the oil temperature.
Stopping performance will be degraded.
Possible damage to bearings, etc.
The dashed lines in Figure
1show the effects of shaft stall on oil temperature and stopping
The possible manoeuvring and machinery problems were considered to be too significant to be
left until Sea Trials and it was decided to attempt to establish and solve the control system and
machinery problems during crash stop manoeuvring on the Shore Trials Facility (STF) at Ansty. Since
most of the problems during the crash stop manoeuvre arise from high power fed into the machinery
system from the ship momentum, it became necessary to simulate a representation of this effect on
the Shore Trials machinery. The system adopted for this purpose has become known as the Power
Injection System.
The principal advantages of the experimental programme associated with Power Injection on the
STF are:
during ship trials, there will be many trials other than those associated with machinery, and
hence time for machinery trials will be at a premium.
if extended machinery trials on the ship were necessary, very substantial costs would be
inevitable.
there will be less scope on the ship than on shore trials for stopping and starting machinery
and for making adjustments and changes to valve settings, instrumentation, control system
parameters, etc.
if the power injection trials produce the required information, then any propulsion system
limitations should at least be known and eliminated or countered by operating procedures
before ship trials This will minimise the time required in the ship for the tuning and setting
to work of machinery and control systems and for machinery trials.
Power Injection System
This paper describes the work involved in progressing the power injection system from its
conceptual requirements through to its final design, implementation and use on the
STF.The initial phase of the study was concerned with the appraisal of the
basic requirements of
power injection and with the investigation of
possible machinery arrangements. Feasibility studies
were performed on the chosen arrangement using computer
simulation techniques; these studies,
which led to the specification of the power injection control system, are described in Section 2.
The formulation and implementation of the computer-based control system are
described in
Section 3. The considerations leading to the choice of a digital computer for the control functions, the
control
and
supervisorysoftware,
associatedhardware
andinstrumentation,
and themachinery/computer interface are detailed in Section 3.
The safety of the machinery under Power Injection conditions was an important factor in the
control system design, and the considerations related to
the allocation of responsibility for plant
safety (test personnel vs computer) and the provision of additional warning and protection devices are
described. A computer model of the system was used to evaluate the dynamic response of the
machinery in fault conditions. These safety aspects are discussed in Section 4.
Prior to installation at Ansty, the Power Injection
control system was checked out using the
computer model of the machinery and the control computer. These tests were mainly associated with
verification of the proper operation of the various computer control modes, and with establishing the
operational requirements for machinery setup and control. To a large extent, these tests were repeated
at Ansty on the real machinery, and both test programmes are described in Section 5.
The Power Injection trials programme to date has followed a series of well-defined test schedules,
each schedule being concerned with particular aspects of machinery and control system performance.
The trials programme and the results to date are discussed in Section 6.
2. BASIC CONCEPT AND MACHINERY REQUIREMENTS
During a crash stop manoeuvre from a high ahead speed, ahead propeller torque derived from the
ship's way is imposed on the transmission system. With the fixed-pitch propeller, this torque,
combined with the inertia torque of the machinery, tends to maintain the ahead rotation of the
machinery. After the shaft reverses and while the ship is still moving ahead, the effect of continuing
propeller feedback is felt as an additional resistance to acceleration of the shaft in the astern rotation.
Thus for astern rotation the system will be operating at higher torque levels than those given by the
astern propeller law. This high torque loading will continue until the ship reaches steady astern
conditions.Figure 2 shows the ship speed, propeller torque and propeller shaft speed transients expected
during a typical crash stop manoeuvre. In terms of propeller loading, the manoeuvre can be divided
into two periods of power absorption separated by a period of power injection. To reproduce these
effects on the Shore Trials Facility over a wide range of crash stop
manoeuvres requires the provisionof controllable load absorption and power injection devices.
The main machinery arrangement on the STF is shown diagrammatically in Figure 3.
Thearrangement is based on a full scale port set of ship machinery and a water dynamometer.
FUEL
C:1
TM3B
Figure 2
AHEAD FLUID COUPLING
AHEAD OIL SUPPLY ASTERN OIL SUPPLY
AND SCOOP CONTROL AND SCOOP CONTROL
Figure 3
$SS CLUTCH
ASTERN FLUID COUPLING
MAIN SHAFT
Time
Controllable Load Absorption
The double circuit water impeller dynamometer is bi-directional and in principle can therefore
provide the required load absorption for ahead and astern rotation. The natural torque/speed
characteristics (obtained with preset control valves) of the dynamometer are not, however,
compatible with the requirements of either the ahead or the astern absorption periods, indicating
that dynamic load control of both ahead and astern dynamometer compartments is essential. The
available controls on each compartment are the water inlet valves and the load control valves,
these valves controlling the inflow and outflow of water respectively. These devices effectively
control the quantity of water retained within each dynamometer compartment at any given
speed, and thereby control the torque exerted on the rotating shaft.
Controllable Power Injection
The power injection source adopted for the system was one of the installed gas turbine units
driving through its ahead fluid coupling, the other gas turbine providing the manoeuvring power.
This arrangement was compatible with the intended usage of power injection, i.e. the simulation
of single-engine manoeuvring conditions, and had the advantage over other proposed devices and
arrangements
of
requiringneither
additional capitalexpenditure nor major machinery
modifications. The arrangement also offered a reasonable expectation of satisfactory control.
It was initially considered that the power injected into the system could be controlled either by
controlling the power output of the gas turbine (via the existing on-engine fuel control system) or
by controlling the power transmitted by the ahead fluid coupling (by scoop trimming control of
the coupling oil lever) or by a combination of both. Subsequent evaluation of the control
requirements showed however that the only effective and controllable method was to control the
gas turbine power output.
To achieve the objectives of the power injection arrangement, a control system was required
which would make use of available controls to continuously adjust the dynamometer load and
the injected power, either independently or in conjunction, to reproduce as accurately as possible
the expected shipboard loading conditions on the manoeuvring engine and transmission.
The basic requirements of the control system were defined in broad terms by analysis of the
power injection requirements, and by knowledge of the capabilities and possible
operatingregimes of the dynamometer and injection engine. To evaluate the detailed requirements and,
indeed, to establish the feasibility and overall controllability of the power injection system, a
mathematical model of the machinery system was implemented on an analogue computer.
The model was formulated such that it could represent both the ship configuration (by the
inclusion of ship motion and propeller equations) and the STF configuration, providing the
feature of almost immediate comparisons between the ship and STF performance at all stages
during the development of the control system.
The power injection control system, devised from experimentation with the computer model
comprised the following sub-systems:
Torque Reference System
Dynamometer Control System
Injection Engine Control System Torque Sharing and Sequencing System Torque Reference System
Since the basic idea of power injection is to reproduce loading conditions on the manoeuvring
engine and transmission on the STF consistent with those imposed by the propeller under equivalent
ship manoeuvring conditions, the load torque imposed by the power injection control system must be
made equal to the propeller torque experienced by the ship machinery under these conditions.
Given identical inertias and frictional losses, this load torque would, if shipboard manoeuvring
procedures are adopted, produce not only identical torque loads on the manoeuvring transmission but
also identical propeller speed transients. The difference between shipboard and STF frictional loads
may be ignored in this context, but the difference in inertias downstream of the fluid
couplings issignificant (-25%), so that considerable deviation from the shipboard shaft speed trajectory would be
expected. There were three courses of action available to overcome the effects of different inertias:
physically increase the downstream inertia on the STF by appending a flywheel on the main shaft
this was ruled out on the grounds of physical dimensions and
cost.suitably modify the torque reference equations so that the load torque imposed is a function of
propeller torque and the inertia ratio
this was ruled out because the required equations
imposed dependency on measurements whose availability and
accuracy were questionable.implement the torque reference system assuming that the inertias are identical and assess both
the degree of difference obtained between the predicted ship and STF manoeuvring transients
and the significance of this difference with respect to the objectives of power injection.
thiswas the course adopted.
The torque reference signal Qp for the control system was obtained from the solution of the
single-degree-of-freedom ship motion equation, the propeller
torque and thrust equations, and the
hull/propeller interaction factors. The solution of the equations inherently formed part of the power
injection control system and pointed the way towards a computer-based control system.
Dynamometer Control System
This system was required to interface with the existing load control devices on the dynamometer,
namely:
outlet water flow control via a servo operated
load control valve (LCV) operating on back
pressure valves (BPV) in the water outflow circuits.inlet water flow control via motorised water inlet
valves (WIV).For steady-state operation, the dynamometer load torque is set by the LCV position, the WIV
position being adjusted to maintain the water temperature below limits. For dynamic load control,
both controls are effective since both influence the rate of change of water content, and hence torque.
To maximise the control effects for increasing loads, high inlet water flows are required; the
converseis true for load shedding control. The
control system adopted the policy of controlling only
on LCVs
with the WIVs for the ahead and astern compartments set for low and high water flows respectively.
The torque control signal to the LCV
servo was developed from a basic open-loop channel, givingthe required LCV position for a given torque demand at a given speed, the loop being closed with
proportional and integral torque error channels.
Injection Engine Control System
The injection engine torque control system was required to interface with the on-engine fuel
control system to produce torque levels on the injection engine consistent with the input torque
demand to the system. The control signal to the on-engine throttle servo was developed from
a basicand the loop was closed with a proportional error channel. Since the throttle servo system is rate
limited, a phase lead circuit was included in the forward loop to improve the cngine response within
the rate limits.
Torque Sharing and Sequencing System
This control system provided the necessary interface between the torque reference system and
the individual torque control loops described above, to ensure that the overall torque requirements are
satisfied and to allocate control responsibility between the dynamometer and the injection engine
during the course of a manoeuvre.
The main problem encountered in the specification of the sequencing system was concerned with
the arrangements for timing the 'entry' and 'exit' points of the injection engine. From Figure 2, the
power injection requirement occurs in the middle phase of the manoeuvre, indicating that the
injection engine is required only during this phase. However, because of the time factors
involved ingetting the injection engine and coupling into the system and capable of supplying
the required
injection power at the time required, there was no alternative to arranging that the injection engine
and coupling be connected and supplying power to the system prior to the start of the crash stop
manoeuvre. In addition, since the dynamometer is not capable of absorbing the required load at low
astern speeds, the period of power injection had to be extended to
compensate. At higher astern
speeds, the dynamometer can absorb the total required astern load, but it was considered that to allow
it to do so would pose the difficulty of deselecting (i.e. taking out of circuit) the
injection engine andcoupling whilst maintaining overall torque control on the system. The most practical solution was to
leave the injection engine in the system until the manoeuvre was complete, and to adopt a load sharing
control policy for this phase of the manoeuvre. In view of the above considerations, an overall control
and sequencing system had to be determined with the injection engine and coupling in circuit and
supplying power to the machinery system throughout the manoeuvre. The system adopted split the
control requirements into three phases (denoted Phases I, H and III), and these are shown in Figure 4.
The phase boundaries and typical machinery transients obtained with the above control arrangements
are shown in Figure 5.
F.1.11F
IMAM
Figure 4
Figure 5
Within the control structure there exists three preset control parameters,
namely:Initial Injection Fuel (IIF) Control
Changeover Torque (COT) Level
Astern Load Sharing (ALS) Control
The COT setting defines the boundary between Phases I and II, and is determined by the torque
level at which the dynamometer becomes uncontrollable. At the
COT value, the injection engine
torque control becomes effective and any remaining load on the dynamometer is dumped. To achieve
a smooth transfer of control from Phase I to Phase II, the IIF control
must be set within a fairly
restricted range. Simulation studies showed that, for each nominal COT setting, there existed a limited
range of possible IIF settings (typically of the order of t5% about the theoretically required setting)
with which the transfer of control could be effected satisfactorily.
IIF settings outwith this range
resulted either in the failure of the main shaft to
reverse (for high IIF settings) or in exceptionally
rapid deceleration of the main shaft (for low IIF settings). It was also noted that for IIFs within the
above range, the system response was more acceptable for settings on the high side, because of the
faster response of the injection engine in reducing power. As an operationai policy, a unique COT
setting was chosen for all manoeuvres, and the IIF control was set some 3 to 5% above the nominal
value required for each manoeuvre, the nominal value being dependent on the manoeuvring power
being used in each instance.
The selection of the IIF settings from the computer studies assumed a basis of identical engines
i.e. that the manoeuvring engine and the injection engine would produce the same power output in
identical conditions. Some difference in performance
characteristics must be expected and the !IF
settings used in practice will have to take account of the engine characteristics obtaining at the time of
the manoeuvres; however the above guidelines still
apply.The function of the ALS control is to allocate the proportioning of the total required astern
ALS was achieved by considering the operation and limitations of the injection engine ahead coupling
when the main shaft is rotating astern. This coupling will be operating at high slips since each side of
the coupling will be rotating in different directions. The power dissipated in the coupling is the power
output of the injection engine plus that proportion of the manoeuvring engine power not absorbed by
the dynamometer. Thus to avoid high power dissipation (and hence high oil temperatures) on the
injection coupling, the proportion of the astern load absorbed by the injection coupling must be
maintained as low as possible. For each manoeuvre, however, there exists a lower limit to the usable
injection
power, below which significant risk to the injection engine is entailed, and which
compromises the control effectiveness. This arises because at low injection engine power levels, the
ahead coupling slip will tend to approach 200% (each side of the coupling running at the same speed
but in opposite directions). Any further reduction in injection engine power (to meet a lower torque
demand) can only result in a speed reduction on the injection engine, thereby increasing the coupling
slip above 200%. The coupling will now be operating in an unstable mode, and torque control becomes
extremely difficult. The minimum torque that the injection engine and coupling can transmit to the
system whilst maintaining the coupling slip below 200% can be calculated for each manoeuvre,
thereby giving the minimum setting for the ALS. Simulation studies indicated that ALS settings in the
range 40 to 60% provided stable and safe operating conditions for all manoeuvres contemplated.
The structure of the control system requirements for power injection, showing the main control
functions and the measurements and control signals involved, is shown in Figure 6.
DRIVING ENGINE Ant COUPLING
0
FUEL PEOG RAM DRIVING ENGINE SSS CLUTCH
A/S COUPLING POWER INJECTION ENGINE
121P
SSS DYNAMOMETER COMPUTER Propeller/Toll s1711.11411011 Dynamometer moue control law Injection torqu control law Torque control sequencingSignal shalting
INJECT/ON ENGINE DYNAMOMETER CONTROL
FUEL DEMAND VALVE POSITION DEMAND
4-36
TRANSDUCERS ACTUATORS
LI
Minuted nrisble 1. Fuel actuator linjectmn engraft,
2. Dynamometer load control valve lensed)
1 Injectlon turbme speed
3. Dynamometer load control valve (astern, Injection P.T. Torque
Dynamometer weed Dynamometer torque
